Due to the advancement of science and continuous development of multimedia, CD-ROM drive has become very popular. The main advantage of CD is high capacity to save data so the speed and the stabilization of CD-ROM drive are very important. When a CD-ROM drive reads data, there is a need for track seeking and track locking. Therefore, the performance of track seeking and track locking will affect the speed and stabilization of CD-ROM drive. In track seeking and locking, radio frequency ripple (RFRP) signal and RFZC signal are very important.
Referring to FIG. 1, that shows a circuit diagram of generating radio frequency zero cross signal. As shown in FIG. 1, an intermediate signal is created by the RFRP through an analog to digital converter. The intermediate signal is calculated through a low pass filter simulated by a digital signal processor (DSP) 20. Then the output of the DSP 20 is transformed through the digital to analog converter DAC 30 to obtain an analog signal. The analog signal is defined as a slice level (V_SL) of the RFRP, and then the RFZC signal is generated from the V_SL signal and the RFRP signal through a comparative amplifier.
Reference is made to FIG. 2, which shows a timing diagram of slice level of radio frequency zero cross signal. In track locking, V_SL is created by passing RFRP signal through a low pass filter. However, a digital signal processor may simulate the function of the low pass filter. When the CD-ROM drive begins, a stable V_SL will be generated after a settling time caused by the low pass filter. The settling time is about forty-six milliseconds.
Reference is made to FIG. 3, which shows a conventional circuit of generating the RFZC signal. As shown in FIG. 3, the comparative amplifier consists of a first resistor 110, a second resistor 112, a third resistor 114, a fourth resistor 116, a fifth resistor 118, a capacitor 120 and an amplifier 122. A terminal of the first resistor 110 is coupled with RFRP signal and another terminal of the first resistor 110 is coupled with a terminal of the second resistor 112. Another terminal of the second resistor 112 is coupled with the positive input of the amplifier 122. The positive input point of the amplifier 122 is further coupled with one terminal of the third resistor 114. Another terminal of the third resistor 114 is coupled with the output of the amplifier 122. A terminal of the forth resistor 116 is coupled with the output of the amplifier 122. Another terminal of the forth resistor 116 is coupled with a power supply (5 volts). One terminal of the fifth resistor 118 is coupled with V_SL and another terminal of the fifth resistor 118 is coupled with the negative input of the amplifier 122 which is coupled with one terminal of the capacitor 120. Another terminal of the capacitor 120 is grounded.
The RFZC signal is generated based on the RFRP signal and V_SL through the comparative amplifier. The main function of the comparative amplifier is to compare the RFRP signal with V_SL for generating the RFZC signal. When V_SL is not correct, the RFZC signal will be incorrect so that the CD-ROM drive will have some wrong or bad performance.
Reference is made to FIG. 4, which shows a diagram of generating the RFZC signal from the RFRF signal and the V_SL in the prior art. The RFZC signal will be different according to different V_SL. In fact, in order to prevent the RFRP signal from noise interference, a region on the timing diagram is defined around the V_SL. The V_SL is in the middle of the region and the region will be changed according to V_SL. In the region, the peak level is a threshold high voltage (V_H) of Schmitt trigger and the bottom level is the threshold low voltage (V_L) of Schmitt trigger. As shown in FIG. 4, the RFZG signal will be positive when the RFRP signal is higher than the V_H level of the Schmitt trigger. When the RFRP signal is lower than the V_L level of the Schmitt trigger, the RFZC signal will be negative. Therefore, the positive cycle and the negative cycle of the RFZC signal are relative to V_SL. In general, it is better for the positive cycle and the negative cycle of RFZC to be symmetric.
How to compensate for the slice level has been disclosed. In that prior art, the disadvantage of the prior method of compensating for the slice level does not respond to the change of the track-crossing signal dynamically. Therefore, the bias voltage will need more time to follow the change of the track-crossing signal when the change of the track-crossing signal is rather violent.
V_SL is used in the track-locking process of the servo system in the optical storage drive (for example, CD-ROM drive, CD-R/RW drive, DVD-ROM drive, DVD player, DVD-R drive, DVD-RW drive, DVD-RAM drive, DVD+RW drive and the like). In general, the DSP unit simulates the function of digital low pass filter to get the V_SL. However, there are two disadvantage of using low pass filter. The first disadvantage is that the settling time is too long and the second disadvantage is that the Schmitt trigger will induce the positive cycle and the negative cycle of the RFZC signal to be asymmetrical. The two disadvantages will affect the performance of the CD-ROM drive and induce the malfunction of CD-ROM drive. Therefore, an effective method is needed to solve this problem.